Seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) of the strata underlying the land surface or seafloor. Among other things, seismic data acquisition involves the generation of acoustic waves, the collection of reflected/refracted versions of those acoustic waves, and processing the collected seismic data to generate the image. This image does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing an improved image of the subsurface in a shorter period of time is an ongoing process in the field of seismic surveying.
A configuration for achieving land seismic data is illustrated in FIG. 1. FIG. 1 shows a system 100 that includes multiple receivers 102 that are positioned over a monitored area 104 of a subsurface to be explored and that are in contact with, or below the surface 106 of, the ground. A number of dedicated seismic sources 108 are also placed on the surface 106 in an adjacent area 110 to the monitored area 104 containing the receivers 102. A dedicated seismic source is defined as a device built by man with the main purpose of generating seismic waves to be used for a seismic survey. As an alternative to being placed on the surface, dedicated seismic sources 108 are buried under surface 106. A central recording device 112 is connected to the plurality of receivers 102 and placed, for example, in a station or truck 114. Each dedicated seismic source 108 can be composed of a variable number of vibrators, typically between one and five, and can include a local controller 116. A central controller 118 can be provided to coordinate the shooting times of sources 108. A global positioning system (GPS) 120 can be used to time-correlate shooting of the dedicated seismic sources 108 and the recordings of the receivers 102.
With this configuration, dedicated seismic sources 108 are controlled to intentionally generate seismic waves, and the plurality of receivers 102 records waves reflected by oil and/or gas reservoirs and other structures and reflection points, e.g., the interface between subsurface formations having different acoustic impedances. The result of the seismic survey contains seismic data for geophysical parameters of subterranean rock formations. The seismic survey records both compressional, or P-waves and shear, or S-waves and is a combination of source wavelet and earth properties.
Analysis of the seismic data interprets earth properties and removes or minimizes the effects of the source wavelet. One type of analysis uses the three dimensional (3D) seismic data to obtain the geophysical properties of the subsurface layer such as P-wave velocity Vp, S-wave velocity Vs and density ρ, which can be used to determine other properties of interest such as impedance, porosity, lithology, fluid saturation as well as other geomechanical properties. This analysis is known as 3D inversion. These properties are used to provide the location and structure of subsurface oil and gas reservoirs. Removal of the oil and gas from the reservoirs and introduction, for example, of water into the reservoirs to aid in the removal of the oil change the geomechanical properties of the reservoir and the location of the oil and gas within the reservoir. Therefore, seismic surveys are taken at different times in order to monitor these changes. Comparison of seismic surveys taken at different times, i.e., vintages, yields data on the changes in the properties of the subsurface layers. The changes in seismic data can be amplitude or time shift or both. For purposes of using inversion to obtain the changes in properties between vintages, time is viewed as a fourth dimension, and the process is 4D inversion.
The initial seismic survey is the base survey and any subsequent seismic survey is a monitor seismic survey. Independent inversion of base and monitor seismic surveys can yield estimates of elastic properties that are inconsistent with expected production effects. Approaches to conducting 4D seismic inversion include inverting base and monitor surveys separately and then differencing the results to calculate changes in elastic attributes, using inversion results for a base survey to define an initial model for inverting a monitor survey, inverting amplitude differences directly for changes in elastic parameters and inverting all vintages simultaneously. This last approach where all vintages are inverted simultaneously is referred to as global inversion. Global inversion of base and monitor is used to obtain quantitative estimates of impedance changes and to reduce the non-uniqueness of the inversion process. The need still exists, however, for improvements in the simultaneous inversion of all base and monitor seismic surveys in order to provide a better determination of the geophysical properties of the subsurface layer such as P-wave velocity Vp, S-wave velocity Vs and density ρ.